Method for Producing a Hot Gas By Oxidation Comprising a Delay Prior to Scavenging

ABSTRACT

The invention relates to an optimized method for producing a hot gas by oxidation of an active material having an oxidized form and a reduced form using a rotary reactor or a simulated-rotation reactor. According to the invention, the production cycle comprises a flow interruption between an oxidation stage and a scavenging stage.

FIELD OF THE INVENTION

The present invention relates to the field of energy production, gas turbines, boilers and furnaces, notably for the petroleum industry, the glass-making industry and in cement plants. The field of the invention also covers the use of these various means for producing electricity, heat or steam.

The field of the invention is more particularly the devices and the methods allowing, through oxidation-reduction reactions of an active phase, to produce a hot gas by means of a hydrocarbon or a hydrocarbon mixture and to isolate the carbon dioxide produced so as to be able to capture it. The invention also applies to the field of hydrogen or oxygen production.

The growing worldwide energy demand leads to build new thermal power plants and to emit increasing amounts of carbon dioxide that are harmful to environment. Capture of carbon dioxide with a view to its sequestration has thus become an imperative necessity.

BACKGROUND OF THE INVENTION

One of the techniques that can be used for capturing carbon dioxide consists in carrying out oxidation-reduction reactions of an active phase so as to decompose the combustion reaction commonly used into two successive reactions:

an oxidation reaction of the active phase with air allows, through the exothermic nature of the oxidation, to obtain a hot gas whose energy can be used,

a reduction reaction of the active phase thus oxidized by means of a reducing gas then allows to obtain a reusable active phase, as well as a gaseous mixture essentially comprising carbon dioxide and water.

The uncoupling thus achieved between the oxidation stage and the reduction stage allows easier later separation of the carbon dioxide from a gaseous mixture practically free of oxygen and nitrogen.

Document U.S. Pat. No. 5,447,024 describes a process comprising a first reactor for a reduction reaction of a metallic oxide by means of a reducing gas, and a second reactor producing said metallic oxide through an oxidation reaction with moistened air. The exhaust gases from these two reactors are fed into the gas turbines of an electric power plant. However, implementation of such a process requires two distinct reactors and transport means for an active phase in form of solid particles. Such a process is therefore relatively complicated to implement and it involves high operation and maintenance costs. Furthermore, fine active phase particles carried along in the exhaust gas can be a source of drawbacks for later treatment of this gas.

Document FR-2,846,710 describes a real rotary reactor in the sense that the reactor exhibits a material rotation between a stationary part and a moving part so as to carry out successive oxidation, then reduction reactions of an active mass.

Document FR-04/08,549 filed by the applicant describes a reactor type allowing to carry out the same reactions as the reactor described in patent application FR-2,846,710, but without a real rotation. Rotation, or more precisely switching from a reactor configuration to another, is obtained through a delay applied, with a fixed periodicity, to a set of modules, preferably identical, each one of which can be supplied by specific means with an oxidizing gas, a reducing gas or an inert gas (referred to as scavenging gas).

These specific means essentially consist of a system of valves allowing to deliver to each module, according to the time period considered, the oxidizing gas, the scavenging gas or the reducing gas. These means are specific to each module.

SUMMARY OF THE INVENTION

The object of the invention is to provide an optimized method for implementing a device allowing oxidation and reduction reactions of an active phase in order to produce a hot gas by means of a hydrocarbon, or a mixture of hydrocarbons, and to jointly isolate the carbon dioxide produced so as to readily capture it.

DETAILED DESCRIPTION

The invention thus relates to a method for producing a hot gas by oxidation of an active material contained by at least one reaction module working as a function of time successively in oxidation, scavenging and reduction stages by contacting through successive circulation respectively of an oxidizing gas, a scavenging gas or a reducing gas.

According to the invention, the contacting stages by circulation of the oxidizing and scavenging gas are separated by a flow interruption of predetermined duration of the oxidizing gas prior to circulating the scavenging gas.

The flow interruption duration can be determined so as to increase the contacting time of the oxidizing gas with the active material.

The rate of circulation of the scavenging gas can be higher than the rate of circulation of the oxidizing gas.

The progress of an oxidizing cycle, then of a reducing cycle on an active mass is reminded hereafter.

The oxidation-reduction cycle of the oxido-reducing active mass comprises a stage of injection of the combustible gas. During this stage, the combustible gas comes into contact with the oxido-reducing active mass that is in a partly oxidized state. The oxygen collected by this mass is transferred to the gas that oxidizes while releasing carbon oxides and water.

The devices according to document FR-2,846,710 or FR-04/08,549 mentioned here by way of reference comprise a set of reaction modules, each module comprising an active material working as a function of time successively in an oxidation, scavenging and reduction stage by contacting respectively with an oxidizing, scavenging or reducing gas. Contacting with the active mass is achieved using either a feed system specific to each module, able to receive as a function of time an oxidizing, scavenging or reducing gas, or a rotary set rotating with respect to a distribution set.

At the start of the fuel injection stage, the gas oxidation reaction (and therefore the reduction of the oxido-reducing mass) is mainly located near the inlet of the module in form of a monolith for example. In the course of time, this oxidation-reduction moves downstream from the module since the upstream oxido-reducing mass has been reduced and therefore no longer contains the oxygen required for combustion.

The applicant has observed a problem relative to unburnt combustible gases. The more the reaction occurs downstream from the monolith, the greater the proportion of combustible gas that can flow through the monolith without having the time to come into contact with the oxido-reducing mass, which increases the volume of unburnt gas.

A simulation was carried out considering a 1-meter long cylindrical channel 2 mm in inside diameter, coated with a 50-μm thick active mass (washcoat). In this oxidation-reduction cycle simulation, the gases are injected at about 500° C., at a pressure of 30 bars. The standard cycle considered comprises the following successive injection durations: 3 s air-0.5 s steam-1.5 s methane-0.5 s steam. The respective injection rates for the air, steam and methane are: 20 m/s, 5 m/s and 1 m/s. The steam is introduced to clean the channel from the oxidizer or the fuel prior to the next introduction of oxidizer or fuel.

In FIG. 1, curve 1 shows the flow rate of combustible gas injected and curve 2 the amount of unburnt gas (in mol/s as ordinate) as a function of time (as abscissa), for a channel. It can be noted that, during the gas oxidation stage (between the times 14.5 and 16 s), a first amount of unburnt gas appears at the channel outlet (curve 2). Then, a “puff” of unburnt gas is produced between 16 s and 16.5 s as illustrated by peak 3.

This “puff” is very harmful to the process yield because it involves a large amount of unburnt gas. Furthermore, it appears concomitantly with a similar carbon dioxide “puff”. Now, according to the method, this CO₂ stream must be directed towards the CO₂ collecting device and not towards the gas turbine. This therefore also applies for this combustible gas “puff” (unburnt gas). The consequence thereof is that a quite considerable amount of unburnt gas has to be sent into the gaseous fluid used for collecting the CO₂. Now, this gas cannot be burnt thereafter since the gaseous fluid selected for CO₂ collection contains no oxygen.

Thus, the reaction yield drops, and CO₂ collection and separation becomes more complicated.

According to the present invention, what is provided is a method or a process that reduces by about 25% the amount of unburnt gas present in said unburnt gas “puff” (peak 3 of FIG. 1). A short period during which nothing is injected is therefore added between the end of the combustible gas injection stage and the next stage of scavenging by steam. The combustible gas trapped in the channel then has the time to oxidize on the oxido-reducing mass, thus reducing the amount of unburnt gas.

FIG. 2 illustrates the new cycle of the process according to the invention. Curve 1 shows as above the rate of injection of the fuel in a cycle and identical conditions for the standard cycle, except for the addition of a 0.5-s injection interruption after injection of the fuel and before injection of the scavenging steam. This pause is shown by reference number 4 in FIG. 2. With this new cycle, curve 2′ shows the unburnt flow rate during the combustion and peak 3′ corresponds to the unburnt gas “puff”.

Numerical Results

The numerical simulations of the method show the efficiency of the solution provided. Table 1 synthesizes the results corresponding to the cycle according to the invention, illustrated by FIG. 2.

TABLE 1 Synthesis of the numerical results According Standard to the in mol/cycle/channel case invention gain/cycle gain/cycle Unburnt gas in the puff 440 10⁻⁶ 325 10⁻⁶ 115 10⁻⁶ 26% (peaks 3, 3′) Unburnt gas during the 467 10⁻⁶ 393 10⁻⁶  74 10⁻⁶ 15% injection of combustible gas (curves 2, 2′) Total unburnt gas per 907 10⁻⁶ 718 10⁻⁶ 189 10⁻⁶ 21% cycle

The last line of the table gives the total amount of unburnt gas for a cycle, i.e. the integral below curves 2+3 and 2′+3′. It can be noted that the cycle according to the invention provides an overall unburnt gas reduction of 21% per cycle. The goal of the invention being however to act upon the volume of said “puff”, the contribution of this invention will be clearer by comparing its volume, according to the invention or according to the standard case. This comparison is shown in the second line of the table. It appears that the invention reduces the volume of unburnt gas of the puff by 26%.

It can also be noted that this better combustion provides a wider use of the oxido-reducing mass, which is translated into an additional 15% decrease on the volume of unburnt gas that has appeared during the combustible gas injection stage (first line in the table).

The present invention advantageously applies to the devices described in documents FR-2,846,710 or FR-04/08,549. 

1) A method for producing a hot gas by oxidation of an active material contained by at least one reaction module working as a function of time successively in oxidation, scavenging and reduction stages by contacting through successive circulation respectively of an oxidizing gas, a scavenging gas or a reducing gas, characterized in that the contacting stages by circulation of the oxidizing and scavenging gas are separated by a flow interruption of predetermined duration of the oxidizing gas prior to circulating the scavenging gas. 2) A method as claimed in claim 1, wherein said duration is determined so as to increase the contacting time of the oxidizing gas on the active material. 3) A method as claimed in claim 1, wherein the rate of circulation of the scavenging gas is higher than the rate of circulation of the oxidizing gas. 